This invention pertains to DC power supplies, and particularly, to such power supplies for converting an input AC voltage to a DC voltage. The present invention is particularly adapted for providing a DC power supply usable in electronic ballasts for fluorescent lighting systems.
The following discussion and description of the invention is directed specifically to such a ballast. However, it is not limited to this use. In fact, it is also applicable to any circuit requiring a power supply having the characteristics provided by this invention.
Electronic ballasts replace earlier core ballasts, achieving greater power efficiency and operability over a wider range of conditions. The advantages of the electronic ballasts are set forth in "Electronic Ballast Improves Efficiency," by R. R. Verderber, Electrical Consultant, November/December 1980, pages 22-26. Examples of prior art electronic ballasts are taught in Stevens, U.S. Pat. No. 4,277,728.
Such previous practice has been to use a standard pulse width modulated (PWM) regulator to control a buck-boost circuit or a buck circuit for generating a DC output voltage from an AC line voltage. These circuits used an input capacitor which was kept small to avoid low power factor and input current waveform distortion. Its actual value depended on the peak power to be handled, the input voltage, and the switching frequency.
Because this input capacitor has to be small, it does not store sufficient energy to prevent the waveform of the rectified line voltage from being effectively a rectified sine wave. This means that a pulse width modulated buck converter cannot draw power until the line voltage reaches the output voltage. This gives dead periods around the zero intercepts of the line voltage during which no line current flows. Such circuits also have to be optimized for operation at a specific line voltage level. They are thus less than optimized at other levels. The converters circuits must also handle higher line voltage peak levels since they are not controlled by the input capacitor. However, in power supplies where input power factor and current distortion are not considered important, a very large input capacitor gives an almost smooth voltage.
It can be seen in such an arrangement that a circuit having a limited sized capacitor is much more prone to damage by rapid line surges, and the like. Radio frequency interference (RFI) is not well filtered. It is also difficult to design a soft start circuit having such an input stage. Further, the energy storage required to prevent excessive ripple of the output voltage must be achieved with other circuit components.
If a PWM regulator has too much gain at the frequency of the ripple voltage on the input capacitor, the output storage capacitor will effectively be connected to the input capacitor, since the duty cycle adjusts rapidly. This causes more conductance when the line voltage is low and less when it is high. The resulting capacitive current flow produces line current distortion and a poor power factor. This is often prevented by setting the DC gain of the PWM system high enough to maintain sufficient load and line regulation, but the AC gain is reduced to zero at input line frequency.
This reduction in AC gain has detrimental side effects. The output voltage is no longer stable against medium frequency fluctuations of the line voltage or the load. Also, if the inductor current does not fall to zero before each switch closure, the line current will not be proportional to the line voltage at each instant of the line cycle.
This latter point can be seen by considering the operation when the inductor current always does fall to zero. In such a case, the PWM operation is at a constant frequency, and over any one line cycle, the duty cycle is constant (due to the low AC gain). While the switch is closed, the voltage across the inductor is a function of the instantaneous line voltage. This translates into a rising inductor current, with the rate of rise dependent upon the line voltage. Since the period that the switch is closed is constant, the amplitude reached is in turn proportional to the line voltage.
When the switch opens, the output voltage appears across the inductor as a back electromotive force. Since the output voltage is nominally stabilized at some constant level, the rate of fall of inductor current is constant. It follows that since the amplitude reached is proportional to the line voltage, and the rate of fall is fixed, the time to reach zero from when the switch opens is proportional to the line voltage.
If each charge of energy put into the inductor is allowed to transfer out to the load before the switch recloses, the next charge will be proportional to the line voltage. However, if charge is still in the inductor when the switch closes, then the new charge drawn from the line will not be in the same proportion to the line voltage. The inductor current and the instantaneous line current will rise to higher levels. This has the effect of causing third harmonic current distortion. Also, if the inductor is larger than a critical value, both the power factor and line current waveforms suffer. Any closure of the switch while inductor current is flowing causes diode losses due to storage delay in the associated current-direction-limiting diode.
If the inductor is smaller than the critical value, the circuit will be less efficient because the periods when no inductor current is flowing have to reflect as higher peak inductor current amplitudes to keep the same power level. This increases the inductor current form factor (RMS/mean) and I.sup.2 R losses. It also puts extra peak current handling requirements onto components, such as the switch, diodes, magnetics, and capacitors.
Even if the designer could ensure that the inductor was exactly the critical value for any given load and instantaneous line voltage, it would be the wrong value for all other conditions. The system has a minimum input voltage variation of zero to approximately 1.414.times.RMS line voltage. When load, component tolerance, and drift variations are also considered, the peak currents experienced are perhaps twice what would occur if the inductor current only just reached zero when the switch recloses.
It is therefore a general object of the present invention to provide a power supply overcoming the just-described shortcomings of the prior art.
In particular, it is an object to provide a DC power supply using current amplitude modulation. Other objects of the present invention are to provide such a power supply which:
a. has increased electrical efficiency due to the elimination of dead periods when no inductor current flows;
b. is optimized for maximum efficiency at all expected levels of input voltage and load;
c. maintains operating efficiency by compensating for major component value changes;
d. resists damage from line transients and rapid surges;
e. overcomes the need for soft start circuits as required for conventional PWM circuits;
f. minimizes ripple in the output voltage while maintaining input power factor;
g. only just lets the inductor current reach zero during each period when the switch is open;
h. does not rely on a fixed time and line voltage relationship to ensure that the line current is proportional to the line voltage;
i. resists damage from high levels of input line voltage;
j. generates a substantially pure sine wave output;
k. provides a thermally-activated safety switch to absorb energy when high voltage surges appear on the input power line and temporarily disconnect the ballast from the power supply;
l. provides a quick response circuit for controlling rapid fluctuations in the output voltage;
m. provides controlled current flow to lamps which are powered by the supply;
n. provides an alternating current output circuit having a computation circuit which compares signals representative of the voltage and current output; and
o. provides a control circuit for controlling the intensity of lamps powered by the supply.